In prior art switching systems comprising a plurality of identical switching nodes, an originating switching node must set up each individual call to a destination switching node via an intervening switching node by involving the intervening switching node in each call control operation such as initiation, progress and termination of calls. One problem with such architecture is that the intervening switching node is substantially performing the same amount of call processing work as the originating switching node and the designation switching node; consequently, the number of calls that can be handled by the intervening switching node is limited. A second problem is that the propagation delay of signaling information for call control is greatly increased because of the need to process that information in the intervening switching node. The effect of these two problems is intensified if more than one intervening switching node is between the originating switching node and designation switching node. Because of these two problems, the prior art architecture of switching systems has not been able to utilize networks of identical switching nodes efficiently, but has been limited to switching nodes interconnected via special purpose hardware (either special purpose switching nodes, e.g., No. 4 ESS, or special center stage switching units, e.g., No. 5 ESS or AT&T Definity Telecommunication Switching System). The prior art has only used switching systems having a plurality of identical nodes when a small number of calls have been interconnected through an intervening switching node and the intervening switching node was also performing an overall switching system control function.
These prior art systems suffer from many problems. The first problem is the high cost of the special purpose equipment (switching nodes or special center stage switching units) and the inability to increase the number of customers served with a small incremental cost per customer as the number of customers increases. The second problem is that it is impossible to have one type of switching node capable of handling from 50 to 8000 customers. The final problem is the need to make the special purpose equipment very reliable since it is the only path between the switching nodes.
Further, the use of ISDN signaling does not provide a direct solution to these problems. ISDN signaling is initially defined by the ISDN standard Q.931 and is intended to provide an international standard to control the initiation of calls, progress of calls, termination of calls, communication of national use information, local serving network information, and user-specific information for telecommunications systems and terminals. The ISDN standard defines two standard links: (1) primary rate interface (PRI) links, each having 23 data channels (B channels) and one signaling channel (D channel), and (2) basic rate interface (BRI) links, each having two B channels and one D channel. The ISDN message format allows each message to have a number of information elements, IE, and groups IEs by codesets. There are eight possible codesets, numbered 0-7. Codeset Q is the current set of information elements and includes IEs that control the initiation, progress, and termination of calls.
In a system having a plurality of switching nodes, the problem with ISDN signaling is that each intervening switching node in the signaling path must process messages communicating the signaling information since each intervening switching node directly terminates the signaling channel. Each intervening switching node processes and interprets all IEs (codeset 0) relating to control of initiation, progress, and removal of calls. The propagation delay of signaling information is greatly increased because of the need to process that information in each intervening switching node, and a large additional processing load is placed on the intervening switching nodes.